Paints are used for a variety of products for purposes of adding attractiveness, protecting a substrate or the like. Among them, protection of a substrate is an important function in particular, and there are demands for paints which can form films having different properties in accordance with various uses. For example, regarding a paint which is used for painting metal products such as elements for automobiles, excellent water resistance and corrosion resistance of a film formed from the paint are required in order to prevent corrosion of a metal substrate due to water and the like.
As a paint which can form a film excellent in water resistance and corrosion resistance, it has been known that a paint which includes a vinyl-modified epoxy ester resin and an organic solvent is excellent in adhesion to a metal substrate and can prevent the corrosion of a metal substrate.
However, a large amount of organic solvent is required to be included in the aforementioned paint which includes a vinyl-modified epoxy ester resin and an organic solvent. Accordingly, from the viewpoint of demands for environmental protection and nature conservation in recent years, development of an eco-friendly water paint, wherein only a small amount of organic solvent is used or no organic solvent is used, has proceeded.
Water paints are paints wherein a resin(s) is dissolved or dispersed in an aqueous solvent. For example, as the method for dispersing a resin in an aqueous solvent, a method using an emulsifying agent can be cited. However, such a water paint wherein a large amount of an emulsifying agent is used has a problem in that the water resistance and the corrosion resistance of a film obtained from the water paint are insufficient.
A method wherein large amounts of hydrophilic groups such as a carboxyl group are introduced into a resin to provide dispersibility can also be cited. However, it is necessary to achieve a high acid value such as 50 or more by introducing a lot of hydrophilic groups to a resin in order to disperse the resin under water sufficiently, and therefore there is a problem in that a film excellent in corrosion resistance cannot be formed using this method.
It has been reported that by using predetermined amounts of an organic solvent in water paint, dispersibility of a resin in an aqueous solvent can be maintained and a film having excellent water resistance and corrosion resistance can be formed, even when the introduced amount of hydrophilic groups and the used amount of an emulsifying agent are decreased. (Please refer to Patent Document 1.) The eco-friendly water paints which industry seeks a paint wherein organic solvent is not included or the quantity of organic solvent is extremely small. However, the reported water paint which includes an organic solvent in combination has a problem such that, when the organic solvent is used in an amount of less than 10% by weight, dispersion stability of a pigment greatly deteriorates, and aggregate are generated by particles dispersing under water and precipitates or the like are generated.
Accordingly, a water paint has been sought which can achieve excellent water dispersion stability even when an organic solvent and large amounts of emulsifying agent are not used. It has been reported that an emulsion-polymerized polymer can be obtained such that an acrylic monomer having a long-chain structure including a carboxyl group represented by the general formula (I) described below is neutralized to be used as an emulsifying agent, and the acrylic monomer is emulsion-polymerized with an α,β-ethylenically unsaturated compound in an aqueous medium to form the emulsion-polymerized polymer; and said emulsion polymerized polymer can show excellent water-dispersion stability even if an organic solvent is not used and a large amount of emulsifying agent is not used. (For example, please refer to Patent Document 2.) However, the aforementioned emulsion-polymerized polymer cannot achieve sufficient water resistance and corrosion resistance of a film obtained from the polymer. Furthermore, dispersion stability of a pigment included in a paint which includes both of the aforementioned emulsion-polymerized polymer and pigment is insufficient, and therefore, there is a problem in that precipitation of pigment is generated with the passage of time.
As described above, various resin compositions have been studied in order to obtain a resin composition which can provide a film excellent in corrosion resistance, and can achieve excellent water dispersion stability even when the quantity of organic solvent or the like used is extremely small. However, such an eco-friendly water paint and a resin composition for water paint wherein the levels required by the industry are satisfied have not been obtained; that is, it has not been possible to reduce the quantity of organic solvent used for a water paint to zero or extremely small, or to obtain excellent dispersion stability of a pigment and a resin and excellent water resistance and corrosion resistance of a film formed by the paint even if such a small amount of organic solvent is used.
Patent Document 1: Japanese Unexamined Patent Application, First Publication, No. 2003-119245
Patent Document 2: Japanese Unexamined Patent Application, First Publication, No. 6-345825